ASKetch
Changes to this app seemed to disable it, but in fact it is better than ever. Make sure you choose the right tool by experimenting with the two pencil types. A swipe gesture will toggle between hard & soft pencils. The hard pencil (squiggly line under)is good for basic drawing and the soft pencil (straight line) is good for fills & textures, but it takes multiple strokes to build up tone. Can produce a water color like effect. The sensitivity to speed and how you end a stoke is quite remarkable. I recommend this program to all of my drawing students.
xray-jr about
Asketch, v1.6.1